In connecting by bolt and nut, a fastened piece is generally bound by a nut in a state of the bolt with an end of the axis a little beyond the upper end of the nut. And the engaged portion between the bolt and nut, being left intact, is inrusted with rust after being exposed to the weather, especially when used outdoors, with result of difficulty in disengagement. For this reason, fastened piece is conventionally adapted to be removed after disengaging the nut from the bolt by means of gas cutting or the like. Consequently, when a bolt is impossible or troublesome to be exchanged, for example, as in a case of stud bolt, a serious influence is given. In case that a bolt and nut are used in a corrosive atmosphere, if the engaged portion is corroded, the proper function of fastening becomes unable to be accomplished.
Even if a fastening portion between the bolt and nut, or a screwed portion is not rusted, in case of a lock nut of all metal adapted to have a plate spring member for locking at an end of the nut and to be shaped to interfere with the screw thread of the bolt by means of said plate spring member, screwing a lock nut onto a bolt causes said plate spring member to be positioned on the side of the free end of the bolt far from the fastened piece, resulting in the plate spring member exposed to the weather or corrosive gases, and the plate spring member is often corroded at first to give rise to failure in function of preventing loosening.